Body tissues, such as tumors, can be destroyed by heat. One way to apply thermal energy to internal body tissue is to focus high-intensity ultrasound waves into the tissue, using a phased array of piezoelectric transducer elements. Such treatment can reduce or even eliminate the need for invasive surgery to remove the tissue. For effective treatment, it is important that a sufficient thermal dose be reached during each ultrasound application (or “sonication”) to ablate, coagulate, or otherwise destroy the portion of the target tissue being heated. Moreover, it is important to avoid painful or damaging heat build-up in healthy tissues surrounding the target tissue. Heating of these surrounding tissues results from the absorption of ultrasound energy outside the focus region, e.g., the absorption of ultrasound by tissue located along the ultrasound beam path between the transducer and the target, or the absorption of ultrasound transmitted through the target by tissues behind the target. Further, heating of the surrounding tissues can result from thermal conduction between the target tissue and nearby healthy tissues.
Non-target tissues can often be protected by allowing cooling periods between successive sonications. Such cooling periods, however, tend to significantly prolong the total treatment time, which can be substantial if numerous sonications are required to fully destroy the target tissue. Since treatment requires conscious sedation and lying inside a magnetic resonance imaging (MRI) apparatus, shortening treatment time is highly desired. To eliminate or reduce the need for cooling periods, non-target tissues can sometimes be actively cooled. For example, during the treatment of superficial tumors, active cooling may be applied to the skin, allowing for faster (i.e., more energy-intensive) treatment of the tumor without overheating the skin. Similarly, other tissue interfaces, such as the rectal wall or endometrium, may be cooled. The resulting temperature gradient between the tumor and the tissue interface achieves the desired protection of non-target tissue. In conventional active cooling, the cooled tissue is typically maintained at a fixed temperature substantially below an estimated threshold temperature for thermal damage, providing a margin of safety.
Depending on the particular anatomy of the region to be treated, active cooling can itself have practical limitations because it can counteract the necessary heating of the tumor; that is, the tumor may experience cooling from a nearby cooled interface, e.g., through direct contact or through blood vessels that go through the interface. As a consequence, a higher ultrasound intensity is required to thermally destroy the tumor. The increased ultrasound intensity, in turn, may expose surrounding tissues that are not amenable to cooling (e.g., due to their location deep inside the body) to thermal damage. Thus, setting the level of cooling involves a trade-off between protecting cooled non-target tissues and limiting the risk to non-target tissue that cannot be cooled. Accordingly, there is a need for improved ultrasound therapy and cooling protocols that provide adequate protection for all non-target tissues while supplying a therapeutically effective dose of thermal energy to the target tissue.